A secondary cell, or a lithium-ion secondary cell in particular, has a small internal impedance and thus has a risk of fire caused by large current at a short-circuit accident. For this reason, voltage ranges of the lithium-ion secondary cell at charging and discharging are strictly regulated, and charging and discharging control is performed by a protection device configured to monitor, for example, voltage, current, and surface temperature, thereby preventing an abnormal operation.
Characteristics of the lithium-ion secondary cell depend on cell deterioration. However, the protection device is unable to diagnose cell deterioration of the lithium-ion secondary cell. For this reason, an accident such as fire of the lithium-ion secondary cell has been occurring despite of the function of the protection device. Prevention of an accident due to cell deterioration requires diagnosis of the cell deterioration and replacement of the lithium-ion secondary cell at an appropriate timing.
A known method of diagnosing cell deterioration of the lithium-ion secondary cell uses a dedicated instrument employing an alternating-current superimposing method. However, this cell deterioration diagnostic method has low versatility because the dedicated instrument is expensive, and furthermore, the lithium-ion secondary cell needs to be removed from an instrument using the cell and connected to the dedicated instrument.
In another known method of diagnosing cell deterioration of the lithium-ion secondary cell, the internal impedance is derived from voltage and current waveforms of the lithium-ion secondary cell in operation, and the cell deterioration is diagnosed based on the internal impedance (refer to Non-Patent Document 1, for example). However, this cell deterioration diagnostic method is not used in practice, because of its high charging rate (SOC) dependency and inaccuracy, for example.